The present invention relates to a semiconductor device, e.g. a thin-film field-effect transistor (TFT), and also to a method for forming it. TFTs are used in VLSIs, liquid-crystal display drivers, and other devices.
Various TFT structures and various methods for fabricating them have been heretofore proposed. A fundamental TFT structure is shown in FIG. 1. This is called the coplanar type and comprises an insulating substrate 101 on which a semiconductor layer 102 is formed. Where the TFT is required to operate at a high speed, a single-crystal semiconductor or polycrystalline semiconductor is used. Similarly to ordinary insulated-gate semiconductor devices, a source region 103 and a drain region 104 which are doped with a dopant to enhance their conductivity are formed by self-alignment, using a gate electrode 106 as a mask. A channel formation region 105 is formed between the source and drain regions. The whole device is coated with an interlayer insulating film 107. The source and drain regions are provided with holes to permit formation of a source electrode 108 and a drain electrode 109. Generally, the depth of the source and drain regions is equal to or less than the thickness of the semiconductor layer 102. This device is not so designed that the crystallinity of those portions of the semiconductor layer which are close to the gate-insulating film is different from the crystallinity of those portions of the semiconductor layer which are close to the insulating substrate.
In the normal TFT structure shown in FIG. 1, the semiconductor layer consisting of a single crystal or polycrystals and showing poor crystallinity is used as the semiconductor layer containing the channel formation region. The semiconductor layer 102 has many defects and, therefore, malfunctions often occur. Slow leakage is a typical example of such malfunction.
That is, where the gate voltage is so low that no channel should be formed, i.e., the gate voltage is lower than the threshold voltage Vth as shown in FIG. 3(B), the relation of the drain current Id to the gate voltage Vg is given by smooth curve A in FIG. 3. Under this condition, i.e., the gate voltage is lower than the threshold voltage Vth, a current flows between the source and drain. It follows that it is substantially impossible to control the drain current by the gate voltage. The spontaneously flowing current at voltages lower than the threshold voltage Vth is known as punch-through current.
This punch-through current flows between the source and drain through a passage that is considerably deeper than the channel surface. Therefore, it is possible to control the punch-through current by increasing the resistance of this passage. However, any feasible TFT having this structure has not been proposed.
It is an object of the present invention to provide a semiconductor device which is free of the foregoing problems including slow leakage.
It is another object of the invention to provide a method for forming such a semiconductor device.
A semiconductor device according to the invention has a semiconductor layer consisting of two portions which differ in crystallinity. One portion has a larger carrier mobility and consists of a semiconductor having a higher crystallinity, e.g. a single crystal or polycrystal semiconductor. The other portion has a smaller carrier mobility and consists of a semiconductor having a lower crystallinity, e.g. an amorphous, semi-amorphous, or microcrystalline semiconductor. The higher crystallinity portion is provided close to a gate insulator of the semiconductor device and the lower crystallinity portion is provided distant from the gate insulator. In this structure, for example, only the shallower portion which will become a channel formation region is crystallized. This is considerably effective in preventing slow leakage.
The fabrication of this semiconductor device starts with formation of a semiconductor layer that is not a single crystal. The surface of this semiconductor layer is illuminated with a first laser radiation or other similar intense radiation to change the semiconductor layer into a single crystal or polycrystals. An insulating film which will become a gate-insulating film is formed. A semiconductor coating is formed only on selected portions of the surface of the insulating film to create a gate electrode. Using this gate electrode as a mask, the laminate is illuminated with a second laser radiation or other similar intense radiation having a wavelength longer than that of the first laser radiation or other similar intense radiation to change the laminate into a single crystal or polycrystals except for both gate electrode and portion underlying the gate.